Modern separation techniques for lanthanides Yield these elements primarily as chlorides or nitrates in aqueous solution. They are used to make other derivatives such as carbonates, hydroxides, oxides, etc. They can also be used to make some of the organic derivatives such as high alkyl carboxylates or acetyl acetonates. Anhydrous lanthanide inorganic salts, however, are essential for making alkoxides, Ln-carbon bond derivatives and a host of others, as well as for producing metals by electrolytic and metallothermic processes. Oxides can be made anhydrous but their low solubility and limited reactivity precludes their use in many areas. The halides are the only class of compounds that are being used as a source of anhydrous species suitable for these purposes. The easiest ones to make are the fluorides. They have, however, only limited use in syntheses due to extremely low solubility and reactivity. The most difficult ones to dehydrate are the iodides. The bromides and chlorides present similar degree of difficulty that is less than the iodides, but greater than the fluorides. In view of economic and environmental considerations, the chlorides are the most sought anhydrous lanthanide salts. This includes cerium, the most abundant among the lanthanides.
The halides which separate from their aqueous solutions usually retain 6-7 moles of water. After the unbound water has been removed, most of the bound water can be removed by careful dehydration below 100.degree. C. It is extremely difficult, however, to remove the last mole of water without decomposing the halide.
Dehydration of lanthanon chloride hydrates may be done with HCl gas, but this is a tedious process. Dehydration using ammonium chloride in addition to HCl, and under temperatures of 200.degree.-300.degree. C., has shown better results. Oxides have been reacted with sulfur monochloride and chlorine or sulfur monochloride alone, and thionyl chloride, carbon tetrachloride and phosgene also been used as reagents. The preparation of the most commonly used anhydrous salt of lanthanides, the chloride salt, is not an easy task. (Chem. Rev. 1962 pp. 503-511)
U.S. Pat. No. 4,492,655 to Gradeff and Schreiber discloses the preparation of Cerium(III) cyclopentadienyl derivatives using Ceric Ammonium Nitrate and Na-cyclopentadienyl. Using a nitrate was a departure from conventional routes. Ceric ammonium nitrate had the advantage of being easily obtained in anhydrous form. The process of that invention consists of slowly adding alkali metal cyclopentadienide to a solution of ceric ammonium nitrate to form in sequence mono to tricyclopentadienyl cerium. The overall reaction equation is shown as EQU 2[Ce(NO.sub.3).sub.4 2NH.sub.4 NO.sub.3 ]+12NaCp.fwdarw.2Ce(Cp).sub.3 +4CpH+(Cp).sub.2 +12NaNO.sub.3 +4NH.sub.3
The "in situ" reduction step is shown to proceed via two possible pathways. EQU Ce(NO.sub.3).sub.4 +4NaCp.fwdarw.Ce(Cp).sub.4 +4NaNO.sub.3 EQU 2Ce(CP).sub.4 +2CpH.fwdarw.2Ce(Cp).sub.3 +Cp.sub.2 +2CpH
The above "in situ" reaction has utility in cases where reagent is a good reducing agent whereby part of the reagent is consumed in the reduction of Ce.sup.+4 to Ce.sup.+3. In cases such as the above referred one, this can be rather expensive.
The purpose of this invention is to provide an anhydrous Cerium(III) species that can be used for the syntheses of other desirable Ce.sup.+3 derivatives which require anhydrous starting material. The objective is to prepare anhydrous Ce(NO.sub.3).sub.3 . NH.sub.4 NO.sub.3 complex. Another objective of the present invention is to provide a convenient process for making this complex which obviates the need for high temperatures and diffcult-to-work-with reagents normally associated with preparation of the anhydrous halides.